1. Field of the Invention
The present invention relates to a group III nitride compound semiconductor device and a method for producing thereof. Particularly, it relates to improvement of a producing method using an epitaxial lateral overgrowth method as a method for growing a group III nitride compound semiconductor film.
The present application is based on Japanese Patent Applications Nos. 2000-189391 and 2000-191780, which are incorporated herein by reference.
2. Description of the Related Art
A so-called epitaxial lateral overgrowth method is known as a method for growing a group III nitride compound semiconductor layer. For example, in the epitaxial lateral overgrowth method as introduced in Unexamined Japanese Patent Publication No. Hei. 10-312971, a mask layer of a growth suppressing material such as SiO2 is patterned on an undercoat by a photolithography method and a wet etching method in advance, so that a group III nitride compound semiconductor layer is grown on a window of the mask, that is, on an exposed portion of the undercoat while a facet structure is formed. As a result, a group III nitride compound semiconductor layer little in crystal defect is epitaxially grown by a metal organic chemical vapor deposition method. For example, a group III nitride compound semiconductor layer formed on a substrate through a low-temperature sedimentary layer is used as the undercoat. The low-temperature sedimentary layer is provided to relax distortion due to difference in thermal expansion coefficient and lattice constant between the substrate and the group III nitride compound semiconductor to thereby reduce crystal defects of the group III nitride compound semiconductor layer formed on the undercoat layer.
According to the aforementioned method in which the formation of the low-temperature sedimentary layer, the formation of the group III nitride compound semiconductor layer as the undercoat layer and the formation of the group III nitride compound semiconductor layer by an epitaxial lateral overgrowth method are performed successively, the group III nitride compound semiconductor layer can be grown with good crystallinity surely.
The temperature for growing the group III nitride compound semiconductor layer by a general metal organic chemical vapor deposition method (hereinafter referred to as “MOCVD” method) is, however, 1000° C. or higher. On the other hand, the growth temperature of the low-temperature sedimentary layer is approximately in a range of from 400° C. to 500° C. Hence, from cleaning the substrate to forming the undercoat layer (group III nitride compound semiconductor layer), the temperature of the substrate changes into high temperature (1000° C.: cleaning of the substrate), low temperature (500° C.: formation of the low-temperature sedimentary layer) and high temperature (1000° C.: formation of the undercoat layer). It is necessary to repeat increase and decrease of the substrate temperature largely. It is therefore, a matter of course that a long time is required for production. Moreover, it is necessary to adjust the substrate temperature in each step. In addition, it is undesirable from the point of view of thermal efficiency.
Therefore, it maybe conceived that the sedimentary layer is formed at a high temperature. However, a bowing problem hereinafter described occurs if the group III nitride compound semiconductor layer (for example, an AlN layer the same as the low-temperature sedimentary layer) is directly grown on the substrate at a high temperature of about 1000° C.
For example, in a light-emitting device having a configuration in which a group III nitride compound semiconductor layer having a device function is epitaxially grown on a surface of a sapphire substrate, distortion occurs between the sapphire substrate and the group III nitride compound semiconductor layer because the thermal expansion coefficient and lattice constant of the sapphire substrate are different from those of the group III nitride compound semiconductor layer. The distortion causes a phenomenon that the laminate of the sapphire substrate and the group III nitride compound semiconductor layer is bowed. If the bowing is too large, there is a disadvantage in alignment control at the time of the production of the device as well as there is a risk that the crystallinity of semiconductor may deteriorate or the semiconductor layer may crack.
The inventors of the present invention have made researches to solve the aforementioned bowing problem. As a result, the inventors have proposed an invention configured as follows:
A group III nitride compound semiconductor device comprising: a group III nitride compound semiconductor layer having a device function; and an undercoat layer having a surface on which the group III nitride compound semiconductor layer can be formed; wherein slopes are formed in the surface of the undercoat layer so that the ratio of the area occupied by the slopes to the surface of the undercoat layer is in a range of from 5 to 100% on a plane of projection.
Further, from another point of view, the undercoat layer is formed as a texture structure. The texture structure used herein means a structure in which the surface of the undercoat layer is shaped like teeth of a saw when any section thereof is viewed, that is, peaks and troughs are repeated through the slopes. The peak portions may contain peak portions shaped like independent polygonal pyramids (inclusive of cones) or peak portions connected like a mountain range.
Further, the trapezoid shape in section used in the above-mentioned invention means a shape in which a flat region on the top of each of the peak portions has a large area. The pit shape used in the above-mentioned invention means a shape in which the flat region has a further larger area.
In the above-mentioned invention, the texture structure shows the case where the ratio of the slope regions to a plane of projection is in a range of from 70 to 100%; the trapezoid shape in section shows the case where the ratio is in a range of from 30 to 70%; and the pit shape shows the case where the ratio is in a range of from 5 to 30%.
When the aforementioned undercoat layer is used, distortion between the group III nitride compound semiconductor layer and the substrate inclusive of the undercoat layer can be relaxed. It is conceived that stress applied on a hetero interface is diffused in parallel to the slopes because of the presence of the slopes in the hetero interface and, accordingly, the stress is relaxed. When distortion is relaxed in the aforementioned manner, the bowing problem is reduced. As a result, the group III nitride compound semiconductor layer can be prevented from cracking. Moreover, the crystallinity of the group III nitride compound semiconductor layer can be improved. In addition, the group III nitride compound layer can be aligned easily when the device is produced.
The inventors of the present invention further made researches upon the undercoat layer having the aforementioned surface structure. As a result, the inventors have found the following problem.
In the embodiment of the proposed invention, examination has been made upon the case where AlN was exclusively selected as the material of the undercoat layer. When an undercoat layer having a texture structure or the like was to be formed from AlN by an MOCVD method, it was necessary to make the inside pressure of the reactor lower than that in the case where group III nitride compound semiconductor layers were grown. Specifically, the inside pressure of the reactor was preferably selected to be in a range of from 50 to 300 Pa, when the undercoat layer was formed, whereas the inside pressure of the reactor was of 1 atmosphere when the group III nitride compound semiconductor layer (generally, an n-type contact layer of n-type GaN) formed on the undercoat layer was grown.
If there was a pressure variation in the film-forming process as described above, an interface before and after the pressure variation was apt to be roughened and it was necessary to control the condition severely to smoothen the interface. Moreover, the film-forming apparatus needed to have a mechanism to make the pressure variation possible. As a result, the size and cost of the film-forming apparatus became large and high. This caused increase of the production cost of the device.